1. Technical Field
The present invention relates to a laser machining apparatus that can provide high precision machining of fine patterns and that is suitable for use in mass production, and to a laser machining method therewith. The present invention also relates to a high quality liquid crystal display panel having an electrode structure that is patterned by this laser machining method.
2. Background Art
There are various types of known laser machining apparatus. These include a CO2 laser apparatus that is a popular technique to cut or bore a metal plate, and a YAG laser apparatus that is broadly used to perform high precision machining of a thin metal sheet. In particular, the YAG laser apparatus is a useful tool for various kinds of high precision machining owing to its small size, easy maintenance, and high performance that can readily provide a laser beam focused into a spot with a diameter of a few tenths of a micron. Another advantage of the YAG laser is that the second harmonic wave(at a wavelength of 532 nm) is available to perform precision machining on a thin film using the ablation effect. The xe2x80x9cablationxe2x80x9d means here an effect that when a macromolecular material is illuminated with a short-pulse laser beam with a short wavelength such as an eximer laser or YAG harmonic laser, the illuminated portion is instantaneously decomposed, vaporized, and scattered, whereby the portion illuminated with the laser beam is removed. In fact, Q-switched YAG lasers are used in various applications such as correction of defects of a semiconductor production mask, patterning of detecting portions of a thin film sensor, patterning of electrodes of a liquid crystal display panel, etc. The Q-switched laser is preferably used in these application fields because it can provide a laser beam with a high peak power for a short pulse duration thereby performing high quality machining on a material to be machined without giving thermal damage. For further detailed information on the ablation machining technique, refer to xe2x80x9cApplication of a Short-Wavelength Short-Pulse Laser to Ablation Machiningxe2x80x9d (The Journal of the Institute of Precision Engineering, Vol.3, pp.473-478 (1993)).
One of important applications of thin film machining techniques is patterning of transparent electrodes of a liquid crystal panel, in which high quality machining and high machining capability are required. In general, electrodes of a liquid crystal panel are patterned by illuminating a transparent conductive film coated on a substrate with a laser beam while moving the substrate relative to the laser beam, thereby cutting the conductive film along lines spaced from each other at a predetermined distance. The machining quality and thus the electrical characteristics of the conductive film depend on the characteristics (mainly the peak power) of the Q-switched YAG laser used. The characteristics of the Q-switched laser in turn depend on the Q-switching frequency. If the Q-switching frequency is lowered, the pulse width becomes narrower, and the peak power becomes greater. Conversely, if the Q-switching frequency is raised, the pulse width becomes wider and the peak power becomes lower.
From the viewpoint of machining quality, it is desirable to use a lower Q-switching frequency to obtain a higher peak power of the laser beam. Then, it is possible to remove a limited portion in an instant via the ablation effect without giving thermal damage to the region near the removed portion and to the substrate on which the film is formed. Machining techniques based on the above method are disclosed in Japanese Laid-Open Patents Nos. 60-261142 and 2-259727. However, these machining techniques have disadvantage in productivity, because the lowering of the Q-switching frequency leads to the corresponding reduction in the moving speed of the stage, which in turn results in an extreme reduction in machining speed.
From the viewpoint of the machining speed, therefore, it is desirable to employ a higher Q-switching frequency to move the stage at a higher speed. However, if the Q-switching frequency is raised, the peak power becomes lower and the pulse width becomes wider. As a result, during the patterning of electrodes of a liquid crystal panel, thermal damage occurs in a glass substrate on which the electrodes are formed. The thermal damage generates microscopic cracks or depressions which may cause degradation in display quality of the liquid crystal panel. Glass contains a small amount of alkali metal. The alkali ions can dissolve into the liquid crystal via the cracks or depressions, which can cause defects in the display of the liquid crystal panel.
Thus, it becomes necessary to divide the beam into a plurality of rays of beam in order to increase the machining speed. Japanese laid-open patent Sho61-89636, discloses a method using a cylindrical lens array. However, this method has several drawbacks as indicated below:
(1) This method is effective only if the incident beams have a uniform intensity distribution. If they are not uniform (e.g., Gaussian distribution), each of the divided beams has a different intensity and the groove width processed via these beams will be varied. For example, the application of this method to the patterning of electrodes of a liquid crystal panel causes local turbulence of the field distribution due to the varied groove width, and nonuniformity occurs in the orientation of liquid crystal. This nonuniformity causes a degradation in the display quality of the liquid crystal panel.
(2) Since the number of lenses has to be the same as the beam dividing number, the size and weight of the entire lens array grow as the beam dividing number increases, which makes it difficult to adjust and hold the arrangement of the lens array. In order to create a beam of approximately 10 xcexcm wide required for micro-fabrication, the numerical aperture of the lens is required to be large. Assuming that the focal length of the lens is 100 mm and the wavelength of the laser is 400 mm, the lens should be 8 mm wide with no aberration.
(3) In the case of (2), focused beam lines approximately 10 xcexcm in width should be arranged at 8 mm intervals if a plurality of 8 mm wide lenses are lined in the form of an array. After a plurality of grooves have been formed simultaneously at intervals of the lens width, the patterning can be completed by moving the stage at desired groove intervals (e.g., 130 xcexcm) and then repeating the process. This method does not allow a plurality of grooves to be formed simultaneously in a region as small as the width of a lens.
(4) Since the intensity distribution on the focused beam lines achieved by a cylindrical lens array is in a slit shape, there is no problem with forming a groove in a straight line. However, it is effectively impossible to form a groove in a curved or slant line.
It is apparent that problems discussed in (1), (2) and (3) are inevitable when using a cylindrical lens array or circular lens array.
To solve the above problems, it is an object of the present invention to provide a laser machining apparatus and method which can provide high machining quality and high machining efficiency. It is another object of the present invention to provide a high quality liquid crystal panel produced by using the laser machining apparatus according to the machining method.
In accordance with the broad teachings of the present invention, laser beams from one or more pulse laser generators are divided into plural beams for simultaneously processing an object. The technique for dividing the laser beams is relatively simple and lends itself to high speed applications.
According to the preferred embodiment of the present invention, there is provided a method of laser machining, including the steps of: driving a plurality of pulse laser generators such that there is a delay in phase of the generating cycle between the plurality of pulse laser generators; making beams generated by the plurality of laser generators equal in quality to each other; and converting the beams generated by the plurality of laser generators into elliptically polarized laser beams.
Thus, the present invention provides the following features or advantages:
(1) It is possible to reduce the pulse frequency of each pulse laser generator without reducing the effective total pulse frequency;
(2) Therefore, it is possible to obtain optimum peak power and an optimum pulse width, thereby performing high quality processing on an object to be processed without introducing thermal damage in it; and
(3) Since an elliptically polarized laser beam is used, it is possible to maintain required high machining quality regardless of anisotropy or nonuniformity of an object to be processed and regardless of foreign particles on the object.
Furthermore, a diffraction grating is used to perform phase modulation on a beam generated by the laser generator thereby dividing the beam into plurality of laser beams. Thus, it becomes possible to simultaneously process a plurality of surface areas of an object to be processed. This allows great improvement in machining capability. That is, if the beam dividing number is assumed to be N, then it is possible to achieve the machining capability N times the capability obtainable using one laser beam.
Using the above laser machining apparatus, and according to the laser machining method of the present invention, a transparent conductive film deposited on a substrate is selectively illuminated with a plurality of laser beams while moving the substrate or the plurality of laser beams, thereby producing a plurality of open grooves in the transparent conductive film at the same time. Furthermore, the means for dividing a laser beam into a plurality of laser beams is realized by a binary phase grating having a two-level relief structure on its surface, which is adapted such that each divided laser beam has equal intensity. With this arrangement, it is possible to produce open grooves having a uniform shape and uniform quality.
The present invention also provides a liquid crystal panel having an electrode structure patterned using the above laser machining apparatus and according to the above machining method. In this liquid crystal panel, grooves (gaps) having a width less than 10 xcexcm are uniformly formed in a display area at constant intervals. Thus, this liquid crystal panel provides a much greater contrast ratio and much better visibility performance. These are primary factors which dominate the display quality. Compared to a conventional technique in which the patterning of electrodes is performed using a photolithography technique, the present invention has an advantage that the patterning can be performed by a smaller number of processes without using wet processing. This allows a reduction in complexity of production equipment, and also ensures that reliable production control can be easily performed. Thus, it is also possible to reduce the production cost of a liquid crystal panel.